The bus cable carries the transmitted message along the cable.
As the message arrives at each workstation, the workstation
computer checks the destination address contained in the message
to see if it matches it's own. If the address does not match, the
workstation does nothing more.

If the workstation address matches that contained in the
message, the workstation processes the message. The message is
transmitted along the cable and is visible to all computers
connected to that cable.

The above diagram shows a number of computers connected to a
Bus cable, in this case, implemented as Thin Ethernet. Each
computer has a network card installed, which directly attaches to
the network bus cable via a T-Connector.

It is becoming common to use 10BaseT (UTP) for implementing
Ethernet LANS. Each workstation is wired in star fashion back to
a concentrator wiring centre (hub). The hub is a multi-port
device supporting up to about 32 ports. One of these ports is
connected to a server, or the output of the hub can be connected
to other hubs.

Multiple access refers to the fact that in bus
systems, each station has access to the common cable.

Carrier sense refers to the fact that each
station listens to see if no other station is transmitting before
sending data.

Collision detection refers to the principle of
listening to see if other stations are transmitting whilst we are
transmitting.

In bus systems, all stations have access to the same cable
medium. It is therefore possible that a station may already be
transmitting when another station wants to transmit. Rule 1 is
that a station must listen to determine if another station is
transmitting before initiating a transmission. If the network is
busy, then the station must back off and wait a random interval
before trying again.

Rule 2 is that a station which is transmitting must monitor
the network to see if another station has begun transmission.
This is a collision, and if this occurs, both stations must back
off and retry after a random time interval. As it takes a finite
time for signals to travel down the cable, it is possible for
more than one station to think that the network is free and both
grab it at the same time.

CSMA/CD models what happens in the real world. People involved
in group conversation tend to obey much the same behavior.

Physical Bus Cable Limits

10Base2 THIN ETHERNET NETWORK LAYOUT
Limitations

maximum number of trunk segments = 5

maximum trunk segment length = 607 feet
(185 meters)

maximum network trunk cable = 3035 feet
(925 meters)

maximum number of stations on a trunk
segment = 30

minimum distance between T connectors =
1.5 feet (0.5 meters)

Rules

each end of the trunk segment is
terminated in 50-ohms

one of the terminators is grounded

connector splices are kept to a minimum

Cabling

BNC-T type connectors

RG58-AU 50-ohm cable, 0.2"

10Base5 THICK ETHERNET NETWORK
LAYOUT
Limitations

maximum number of trunk segments = 5

maximum trunk segment length = 1640 feet (500 meters)

maximum network trunk cable = 8200 feet (2500 meters)

maximum number of stations on a trunk segment = 100

minimum distance between transceivers = 8 feet (2.5
meters)

maximum transceiver cable length = 165 feet (50 meters)

Rules

each end of the trunk segment is terminated in 50-ohm

one of the terminators is grounded

connector splices are kept to a minimum

Cabling

Transceivers 802.3

50-ohm cable RG-11

Male DIX connector

Wiring of the DIX Connector

Pin

Ethernet

IEEE 802.3

1

Shield

Control-in

Shield

2

Collision presence+

Control-in

A

3

Transmit+

Data-out

A

4

Reserved

Data-in

Shield

5

Receive+

Data-in

A

6

Power return

Voltage

common

7

Reserved

Control-out

A

8

Reserved

Control-out

Shield

9

Collision presence-

Control-in

B

10

Transmit-

Data-out

B

11

Reserved

Data-out

Shield

12

Receive-

Data-in

B

13

Power

Voltage

14

Reserved

Voltage

Shield

15

Reserved

Control-out

B

Shell

---

Protective

Ground

10BaseT UTP NETWORK LAYOUT
Limitations

maximum segment length of 100 Meters

Hub to Hub or repeater to repeater links limited to 100
Meters

Rules

star topology

4 repeater/5 segment rule of 10Base5 is retained

only two nodes per segment are allowed

Cabling

RJ-45 Connectors

Category 3 UTP minimum, preferably Category 5

Bus Network Topology Summary

Advantages

Disadvantages

Easy to implement

Limits on cable length and Workstation numbers

Low Cost

Difficult to isolate network faults

A cable fault affects all workstations

As the number of workstations increase, the speed of
the network slows down